For such a famous man, we actually know relatively little about him for sure. However, his legacy as a mathematician and philosopher has had a huge impact on Western culture in many fields. It is a pity most people today just know him for his eponymous triangle theorem, important as it is.

Pythagoras in Raphael's School of Athens

Trigonometry

Angles of right-angled triangles may be calculated using trigonometric functions. The definitions of the three basic trig functions are:

Sine

The sine of an angle is equal to the length of the opposite side over the hypotenuse: $sinB = b/c$

If two sides of a triangle are known, it is possible to calculate the angle by in the inverse sine function: $B = sin^{-1}({b/c})$.

Cosine

The cosine of an angle is equal to the length of the adjacent side over the hypotenuse: $cosB = a/c$

If two sides of a triangle are known, it is possible to calculate the angle by in the inverse cosine function: $B = cos^{-1}({a/c})$.

Tangent

The tangent of an angle is equal to the length of the opposite side over the adjacent side: $tanB = b/a$

If two sides of a triangle are known, it is possible to calculate the angle by in the inverse tangent function: $B = tan^{-1}({b/a})$.